A modular cable unit for oilfield wireline includes multiple cable modules. The cable modules are interchangeable to achieve a modular cable unit with desired telemetry and electrical properties to suit a specific application. The cable modules can be an optical fiber module, a power cable or an opto-electrical module assembly. The cable modules that make up the modular cable unit are preferably arranged in a triad configuration defining a substantially triangular tangent periphery and are surrounded by a polymeric casing having a circular periphery. The triad configuration of the modular cable unit contributes to an improved mechanical strength. A floating-tube type optical fiber element with improved mechanical strength is also disclosed.